In the state of the prior art, nowadays and especially in goods vehicles, permanent braking units (wear-free brake systems) are used to relieve the load on the operating brakes. Permanent braking units can also improve the economy of goods vehicles by allowing higher average speeds (especially on long downhill drives) and by considerably reducing the wear of the brake linings of the operating brakes.
Engine brake systems are used as permanent brakes which, during a thrust operation, produce a braking torque that depends on the gear engaged. In addition, retarder brakes are known to convert kinetic energy into heat energy, which differ in their manner of energy conversion in that with hydrodynamic retarders, the energy is converted by fluid friction and, with electrodynamic retarders, by means of a magnetic field. Retarders act as virtually wear-free permanent brakes, especially for goods vehicles and locomotives, since they have the advantage of converting the energy to be braked into heat without wear over long periods of time.
In hydrodynamic retarders, the energy flow of a fluid is utilized for braking, the physical action principle corresponding to that of a hydrodynamic clutch with fixed turbine. According to this, a hydrodynamic retarder comprises a rotor in the fluid flow path and a stator fixedly attached to the retarder housing. When the retarder is actuated, a quantity of oil corresponding to the desired braking performance is introduced into the turbine blade area, which the rotor impels against the stator so that a braking action is exerted on the rotor.
In electrodynamic retarders, on the other hand, the principle used for braking is that of the action of force in electromagnetic fields. Here, a stator is provided with several energizing coils and attached to the transmission housing. Air-cooled rotors are also provided on the transmission-gear side, which are usually connected to the drive shaft. For braking, the energizing coils are supplied with current. As the rotors pass through the magnetic field, eddy currents are induced which impede the rotary movement of the rotors.
Depending on their arrangement in the drive train, retarders are divided into primary and secondary retarders, primary retarders being arranged on the engine side and secondary retarders on the transmission side. In the present state of the art, electrodynamic retarders are mainly arranged as secondary retarders. This means that primary retarders operate as a function of the engine speed, while secondary retarders operate as a function of the speed of the vehicle.
In addition, permanent braking systems are divided into stepped and infinitely-variable systems; stepped systems are the engine brakes and the electrodynamic retarders. In contrast to the infinitely-variable systems, such as hydrodynamic retarders, the braking performance can only be varied in steps.
Permanent brakes are particularly important in the case when the speed must be kept constant downhill, but this often entails discomfort for the driver.
At lower drive shaft speeds, hydrodynamic secondary retarders have their system limits, i.e., the braking torque produce is no longer sufficient. In addition, with hydrodynamic secondary retarders and, as a rule at high drive shaft speeds, the power is limited or reduced in order to protect the engine cooling system.
Accordingly, the purpose of the present invention is to indicate a method for maintaining a desired braking torque with optimum utilization of the wear-free braking systems of a motor vehicle.
For this, it is proposed to combine the strengths of the available braking systems (e.g., hydrodynamic+electrical secondary retarder, hydrodynamic secondary retarder+engine braking). In this way the weaknesses of the available various respective braking systems can be compensated.